1. Field of Invention
The invention relates to a moving target indicating (MTI) radar system and more particularly to the reduction of the masking effect in the doppler frequency domain of wind sheared clutter upon the radar returns of moving targets.
2. Description of the Prior Art
"Wind sheared" weather and chaff are long standing radar problems, particularly for long range ground based radar. Wind shear is by definition a component of the wind for which the velocity changes with altitude. It is a world-wide and year round phenomenon. Wind shear is the velocity spread of wind borne weather (rain, hail, snow) and chaff particles across a vertical section of the main beam of the radar. This velocity spread causes the doppler spectrum of these particles, which cause radar reflections called "clutter", to spread in the frequency domain and thereby mask a substantial portion of the doppler frequency domain. (The unambiguous doppler frequency domain associated with a pulse--doppler radar is the frequency domain bounded by zero and the PRF (the pulse repetition frequency, which is the reciprocal of the pulse repetition period, T) and through which the doppler frequency induced by the radial speed of the target may range from zero and a maximum established for the system (usually the PRF). In anticipation of weather or chaff, an N-pulse MTI filter may be provided to reject the clutter. Such a filter attenuates both the clutter and any targets having speeds causing joint occupancy of the doppler frequency domain. If for example the clutter occupies 30% of the doppler frequency domain, the unattenuated frequency domain remaining for target detection is limited to 70%.
The doppler frequency spread due to wind shear increases with the frequency of the radar system. In addition, the radar cross section of the weather particles and chaff increase with frequency. Thus, the adverse effect of wind shear increases with RF frequency.
The effect of wind shear also increases with range. The fractional doppler occupancy f.sub.d T (i.e. the doppler frequency spread of the clutter across the main beam multiplied by the pulse repetition period) and denoted here by .nu., may be calculated, assuming a wind shear constant of 4 meters per second per kilometer of altitude directed towards or away from the radar and assuming a 1.degree. radar beam, set at 0.degree. elevation. Calculations indicate that the fractional occupancy .nu. is small in the 50 kilometer range for the 10, 5 and 3 gHz radar bands, being less than 8% at 10 gHz. At 100 kilometers range the fractional occupancies are respectively 30%, 15% and 9%, and thus of serious concern in all three bands. At 150 kilometer range intervals, the clutter occupancy approaches unity at higher frequencies. At longer ranges, the clutter occupancy exceeds unity-meaning that there may be no clutter free doppler frequency domain--even at lower frequencies. In short, wind shear poses a serious problem to all weather radar operation, and means for countering its effect are of great importance.